EP0325640B1

EP0325640B1 - Raman cavity dump laser

Info

Publication number: EP0325640B1
Application number: EP88906814A
Authority: EP
Inventors: Robert D. Stultz; Donald E. Narhi
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1987-08-10
Filing date: 1988-07-05
Publication date: 1994-01-05
Anticipated expiration: 2008-07-05
Also published as: JPH02500943A; NO178515C; NO891347D0; GR880100480A; EP0325640A1; WO1989001715A1; IL87074A; TR26676A; NO178515B; ES2008560A6; EP0325640B2; GR1001114B; KR890702305A; IL87074A0; EG18675A; KR920009707B1; DE3886915D1; DE3886915T3; DE3886915T2; NO891347L

Abstract

A lasing medium (3) and a Raman medium (20) share a common optical cavity. The lasing medium (3) projects laser light into the Raman medium (20) and, when a threshold intensity within the Raman medium is reached, the Raman medium absorbs the laser light and re-radiates coherent light at a shifted frequency. Optical elements within the system provide an optical cavity for the lasing medium and a second cavity for the Raman medium.

Description

1. Field of the Invention

The invention concerns lasing systems and, more specifically, a system in which the Q of a resonant cavity is spoiled by the use of the stimulated Raman effect, without using a conventional shutter.

2. Discussion

As shown in FIG. 1, a typical laser includes a lasing medium 3 positioned between two mirrors 6 and 9. The spacing 12 between the mirrors is an integral number of half wave lengths of the laser frequency. That is, distance 12 equals (N) (lambda)/2, where N is an integer and lambda is the wave length. Therefore, mirrors 6 and 9 form a cavity which can support standing waves at the wave length lambda.
Q is a figure of merit which refers to the sharpness of the resonance in the cavity. A technique called Q-switching is frequently used to obtain short, intense bursts of light from the laser cavity.
In Q-switching, a shutter 15 closes and obstructs one of the mirrors, mirror 9 in this case, thereby preventing photons from travelling back and forth between the mirrors. The closed shutter prevents oscillations by reducing (or spoiling) the Q of the cavity. Without oscillation, stimulated emission is inhibited, and increasing population inversion in the lasing medium 3 is promoted. When the shutter 15 is opened, the cavity becomes high-Q, and reflections between mirrors 6 and 9 resume, thereby stimulating emission and allowing a depletion of the population inversion. The depletion is rapid, resulting in a brief, intense pulse of light.
Several types of shutter can be used. Electro- optic, magneto-optic, or acousto-optic modulators can be used, as well as a bleachable dye which becomes transparent in the presence of sufficient photons. For example from US-A- 3743964 a laser is known that uses as the Q-switch a saturable absorber dye from the class of bivalent transition metal 2 ditheine complexes having a square planar structure. Also, a rotating mirror can be used. However, all of these shutters, with the exception of the dye switch, tend to be expensive. As to a dye switch, some environmental conditions can prohibit the use of dye Q-switches.
Researchers have used Raman cells with Q-switching to generate brief light pulses. For example, R. Frey, A. deMartino and F. Pradere, in "High-Efficiency Pulse Compression With Intra-Cavity Raman Oscillators," Optics Letters, Volume 8, Number 8, Page 437, August, 1983, discuss the use of intra-cavity Raman cells to generate short pulses at the first Stokes frequency. In addition, F. deRougemont, Ding Kong Xian, R. Frey, and F. Pradere, in "High Efficiency Pulse Compression With Externally Pumped Intra-Cavity Raman Oscillators", Optics Letters, Volume 9, Number 10, Page 460, October, 1984, discuss the use of a high Q resonator at the first Stokes frequency used to generate short pulses at the second Stokes frequency. However, these researchers all use a conventional Q-switch in their apparatus.

OBJECTS OF THE INVENTION

It is an object of the present invention to provide an improved lasing system.
It is a further object of the present invention to provide a lasing system that produces short light pulses without using a conventional Q-switch.
It is a still further object of the present invention to provide a lasing system that produces light at a frequency which is shifted from the laser frequency.

SUMMARY OF THE INVENTION

This object is solved by a laser system according to claim 1 and a method of spoiling the Q of a laser cavity according to claim 13. In one form of the invention, two reflectors form a resonant cavity. The cavity contains both a lasing medium and a Raman medium. The lasing medium produces laser photons which pass through the Raman medium. When the intensity of laser photons reaches a threshold (which is characteristic of the Raman medium used), the Raman medium absorbs the laser photons, thereby depleting the laser photon population. At this time, the Raman medium re-radiates light, but at frequencies shifted from the laser frequency, termed the Stokes frequencies. Further, the re-radiated light is coherent.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates a conventional laser.
FIG. 2 and 3 illustrate one form of the present invention, but in two different phases of operation.
FIG. 4 is a plot of output intensity-versus-time of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 and 3 illustrate one form of the invention. The components of each figure are identical, but different components are emphasized in each in order to illustrate two different phases of operation of the invention.
In FIG. 2, a laser medium 3, Which is a Nd:YAG rod of diameter 4.3 millimeters and length 45 millimeters, is coated on surface S₁ for a reflectivity of approximately one hundred percent and coated on surface S₂ to be anti-reflective, both coatings effective at 1064 nanometers. Reflector 18 is a BK7 optical flat coated on surface S₃ to be approximately one hundred percent reflective at 1064 nanometers. Also, reflector 18 is coated so that its total reflectivity, including those of both surfaces S₃ and S₄, is approximately twelve percent (or less) at a different wave length namely, 1542 nanometers. The significance of this latter, smaller reflectivity at the longer wave length will be explained later.
The optical distance between surface S₁ and surface S₃ is 48.7 centimeters. The arrangement just described provides a resonant cavity at 1064 nanometers with surface S₁ and S₃ acting as reflectors, to support lasing of the laser medium 3. This cavity will be termed the S₁ S₃ cavity.
The lasing medium 3 is pumped with a xenon flash lamp using an LC pulse discharge circuit (lamp and circuit not shown.) The value of L is 25 microhenrys and the value of C is 25.3 micro- farads.
Upon firing of the flash lamp, the S₁ S₃ cavity oscillates at 1064 nanometers. However, when the intensity of laser light reaches a threshold, a Raman medium 20 becomes active, as will be explained in connection with FIG. 3.
In that figure, reflector 22, which is a BK7 optical flat coated for reflectivity of about two percent at 1064 nanometers and in excess of ninety- eight percent at 1542 nanometers, cooperates with reflector 18 to provide a low Q-cavity at 1542 nanometers. A surface on reflector 22 is designated S_s, so now the low Q-cavity can be termed the S3S5 cavity. As stated above, the reflectivity of the reflector 18 at 1542 nanometers is less than twelve percent. This causes reflectors 18 and 22 to form a low Q-cavity because most of the photons striking reflector 18 at 1542 nanometers are not reflected, but transmitted as shown by arrow 24.
Contained within this low Q-cavity is a Raman medium in the form of methane (CH₄), in a container pressurized at about 69 atmospheres. The left window 26 is a +63 millimeter focal length BK7 meniscus lens which is anti-reflective coated at both 1064 and 1540 nanometers. This meniscus lens 26 serves to focus the incoming laser light in the methane as shown by waves 28. The right window 30 is a BK7 optical flat, anti-reflective coated at both 1064 and 1542 nanometers. A recol- limating lens 33 is a +50 millimeter focal length BK7 meniscus lens anti-reflective coated at both 1064 and 1542 nanometers. This latter lens serves to recollimate photons exiting from the Raman cell as shown by waves 37.
When the intensity threshold for the Raman medium 20 is reached, stimulated Raman scattering occurs, resulting in the production of photons 40 at the first Stokes frequency which is a characteristic of the Raman medium. In this case, the wave length is 1542 nanometers, computed as follows. The vibrational frequency for methane is 2914 centimeters -¹. The laser wave length of 1064 nanometers converts to a frequency of 9399 centimeters -^1. The difference between these two frequencies, namely, 6485 centimeters -¹, corresponds to a wave length of the first Stokes line at 1542 nanometers.
Figure 4 shows an exemplary oscilloscope trace of the device output. The plot is of intensity-versus-time of radiation at 1542 nanometers. An indium gallium arsenide photodiode detector, together with a storage oscilloscope having a 400 megahertz bandwidth was used to record the pulses. The average output energy was measured to be about two millijoules, using a germanium photodiode detector.
Several important aspects of the invention will now be discussed.

1. The invention provides high-intensity light pulses without the use of conventional Q-switching. An energy of about two millijoules was stated above and the pulse duration is approximately 3.5 nanoseconds, as shown in Figure 4. Further, the output is at a wave length (1542 nanometers) shifted from the primary laser wave length (1064 nanometers).
2. Single pulses were obtained in response to single flashes of the flash lamp, as described in connection with Fig. 4. The inventors believe that it is possible to obtain a train of pulses by substituting a continuous wave (CW) laser for laser 3 in FIG. 3 and 4. In this case, when the stimulated Raman threshold is reached, the Raman medium 20 will rapidly deplete the laser photon population, and will radiate at the Stokes frequency. Following this, a length of time will be required to restore the laser photon population to the Raman threshold, at which time, absorption will again occur, producing a continuous train of pulses at the first Stokes frequency.
3. The invention, in at least one aspect, operates in a fundamentally different manner than conventional Q-switched lasers. To explain this difference, Q-switching will be explained in an oversimplified manner.

In Q-switching, energy storage occurs in the form of a population inversion of electrons. That is, the electrons are promoted to one (or several) higher energy levels. Then, stimulated emission occurs when the cavity is switched to high Q. However, it is clear that a sufficient population inversion must exist at the time of Q-switching. Restated, the rate of spontaneous transition, or decay of the electrons, must be low enough than a sufficient number remain in the high-energy state at the time of Q-switching.
In the present invention, Stimulated Raman Scattering occurs when the laser photon population (not the population of promoted electrons in the laser medium) exceeds the Raman Threshold. Therefore, lasing media can be used having a faster spontaneous decay rate than the ND:YAG described above. The mechanism of maintaining a population inversion until stimulated emission occurs is not dominant in the invention, as it is in Q-switching.
Viewed another way, in Q-switching, laser output is obtained when the Q of the cavity is high. In the present invention, output is obtained when the Q of the S₁ S₃ cavity is low.
An invention has been described in which a high Q-cavity (the S₁ S₃ cavity) contains a low Q-cavity (the S₃S₅ cavity.) The high Q-cavity is tuned to one wave length, 1064 nanometers, while the low Q- cavity is turned to a longer wave length, 1542 nanometers. A Raman medium is contained within the low Q-cavity. A laser is contained within the high Q-cavity, and the laser, the high Q-cavity, and the low Q-cavity all share a common optical path.
When the laser is pumped, the high Q-cavity causes the laser photon population to increase. During this increase, the Raman medium is effectively transparent to the laser photon population. However, when the photon population reaches a threshold, the Raman medium absorbs laser photons and re-radiates them at a shifted frequency, according to the Stimulated Raman Effect. The absorption by the Raman medium spoils the Q of the high Q (S₁ S₃) cavity.
Numerous substitutions and modifications can be undertaken without departing from the scope of the present invention as defined in the following claims:

Claims

1. A laser system comprising:

a first laser means (3, S₁ S₃) comprising a lasing medium (3) which provides first photons at a first frequency, and a high Q-cavity (S₁ S₃) at said first frequency, and

a second laser means (20, S₅S₃) comprising a Raman medium (20) which provides for second photons at a second frequency, and a low Q-cavity (Ss S3) at said second frequency, said Raman medium

(i ) receives said first photons from the said lasing medium (3),

(ii) is substantially transparent with respect to the first photons as long as the photon intensity is below a threshold, and which

(iii) absorbs said first photons when the photon intensity exceeds the threshold and, in response thereto, re- radiates said second photons; wherein

said second laser means (20, S₅S₃) is arranged within said first laser means (3, S₁ S₃).

2. The laser system according to claim 1 in which said Raman medium (20) comprises gaseous methane.

3. The laser system according to claim 1 or 2, further comprising

a lens (26) for focusing laser light (28) from said lasing medium (3) into said Raman medium (20);

a collimating lens (33) for collimating light received from said Raman medium (20); and

a reflector (22) for preventing travel of photons produced by stimulated Raman scattering toward said lasing medium (3).

4. The laser system according to one of the previous claims, in which said low Q-cavity (Ss S3) has a reflector (22) of low reflectivity at said first frequency.

5. The laser system according to one of the previous claims wherein said high Q-cavity (S₁ S₃) and said low Q-cavity (S₅S₃) are positioned along a common optical path.

6. The laser system according to one of the previous claims comprising a first reflector (18) which is reflective at said first frequency and substantially transparent at the second frequency; and

a second reflector (22) which is substantially transparent at said first frequency and substantially reflective at said second frequency.

7. The laser system according to one of the previous claims wherein said first frequency is 1064 nanometers and said second frequency is 1542 nanometers.

8. The laser system according to one of the claims 3 thru 7 wherein said focusing lens (26) and/or said collimating lens (33) is substantially transparent at both frequencies.

9. The laser system according to one of the previous claims wherein said Raman medium (20) contains methane at a pressure of approximate 69 atmospheres.

10. The laser system according to one of the claims 6 thru 9 wherein said first reflector (18) is nearly 100 % reflective at the first frequency and less than 12 % reflective at the second frequency, and wherein a first surface (Si) of said lasing medium (3) and a surface (S₃) of said first reflector (18) form said high Q-cavity (S₁S₃).

11. The laser system according to one of the previous claims wherein said Raman medium (20) absorbs said laser photons at the first frequency when the laser photon intensity exceeds said threshold, thus substantially depleting the photon population and emitting Raman- shifted photons at said second frequency which pass through said Raman medium (20).

12. The laser system according to one of the previous claims comprising the following elements positioned along an optical path in the following sequence:

a first surface (S_i) which is reflective at the first frequency, said lasing medium (3),

a second surface (S₂) which is substantially transparent at the first frequency,

a medium (22) which is reflective at the second frequency and substantially transparent at the first frequency,

said focusing lens (26) which is substantially transparent at both frequencies,

said Raman-cell (20) containing methane at a pressure of approximate 69 atmospheres, a medium (30) which is substantially transparent at both frequencies,

said collimating lens (33) which is substantially transparent at both frequencies,

a medium (18) which is nearly 100 % reflective at the first frequency and less than 12 % at the second frequency, wherein said first surface (Si) and said medium (18) form said high Q-cavity (S₁ S₃) for the lasing medium (3).

13. A method of spoiling the Q of a laser cavity, comprising the steps of

a) providing a lasing medium (3) emitting first photons at a first frequency with a high Q-cavity (S₁ S₃) at said first frequency;

b) providing a Raman medium (20) emitting second photons at a second frequency with a low Q-cavity (S₅S₃) at said second frequency;

c) arranging said low Q-cavity (S₅S₃) within said high Q-cavity (S₁ S₃); and

d) directing said first photons to said Raman medium (20) which is substantially transparent with respect to said first photons as long as the photon intensity is below a threshold and which absorbs said first photons when the photon intensity exceeds the threshold, re- radiating said second photons thereby.